The present invention generally relates to films useful as tape backings, and more particularly to biaxially oriented backings comprising a blend of isotactic polypropylene with at least a second polyolefin.
Commercially available pressure sensitive adhesive tapes are usually provided in a roll form and can be provided on a tape dispenser (see e.g. U.S. Pat. Nos. 4,451,533 and 4,908,278). In the case where no dispenser is provided, it is desirable that the tape can be torn by hand.
Commercially available tape dispensers typically have either a metal or plastic serrated cutting blade. xe2x80x9cSeverabilityxe2x80x9d of adhesive tape is defined as the ability to cut or sever a length of tape by pulling the tape over the teeth on the serrated cutting edge of a tape dispenser with a desired amount of energy or work. Severability is also referred to as xe2x80x9cdispensability.xe2x80x9d It is desired that the severed tape does not chip, sliver, fracture or break in an unpredictable manner (see U.S. Pat. Nos. 4,451,533 and 4,908,278). Such severability is desirable to produce a cleanly serrated cut edge on the severed tape strip.
Commercially available biaxially oriented isotactic polypropylene films are well known for their toughness, low cost, good color, moisture and slivering resistance, and clarity and have long been used as adhesive tape backings (see U.S. Pat. Nos. 3,241,662 and 3,324,218). However, adhesive tapes produced from such films tend to be too tough and stretch significantly before breaking while under load, and this renders such films extremely difficult to sever, particularly on a plastic bladed dispenser.
Commercially available tape dispensers with plastic cutting blades are commonly used for hand-held units such as a Catalog No. 104 3M Magic(trademark) tape dispenser, available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. However, dispensers with plastic cutting blades are typically not sharp and durable enough to sever oriented polypropylene tapes acceptably. For this reason, commercially available dispensers for biaxially oriented polypropylene tape usually are equipped with sharp metal blades. Such metal blade dispensers are more costly and difficult to manufacture than dispensers with plastic blades.
Several attempts at providing desirable biaxially oriented polypropylene films are known from the art. See, for example, U.S. Pat. Nos. 4,414,261, 4,451,533; 5,252,389; 5,118,566 and 5,366,796. Several attempts at providing tape backings that may be torn by hand (typically in the transverse direction of the backing) are known from the art. See for example, U.S. Pat. Nos. 3,491,877; 3,853,598; 3,887,745; 4,045,515; 4,139,669; 4,173,676; 4,393,115; 4,414,261; 4,447,485; 4,513,028; 4,563,441; 4,581,087; 5,374,482; and 5,795,834.
In addition, there have been attempts to provide oriented polymer films comprising polypropylene blended or laminated with at least one other polymer to produce packaging tapes and films. It has been known to provide a packaging adhesive tape based on polypropylene stretched film containing a soft or rubbery polymer such as polyethylene, polyisobutylene, or ethylene propylenediene and stretched primarily transversely to the machine direction. U.S. Pat. No. 4,137,362 discloses an example of a packaging adhesive tape. It has also been known to provide packaging films comprising oriented polypropylene blended or laminated with at least one other polymer and stretched to provide significant heat shrinkage in the longitudinal direction, and also to provide toughness, tear resistance, puncture resistance, or heat sealability. See for example U.S. Pat. Nos. 4,230,767; 5,691,043; 5,292,561; 5,073,458; 5,077,121; and 5,620,803.
It is desired that severable adhesive polyolefin based tapes display a combination of characteristics, particularly low puncture resistance, low tear resistance, low elongation to puncture failure and low tensile strength in the longitudinal tape direction. It has been surprisingly found that biaxially oriented adhesive tape backing comprising isotactic polypropylene combined with at least one other polyolefin can be biaxially stretched so as to provide the desirable properties described herein.
Certain terms are used in the description and the claims that, while for the most part are well known, may require some explanation. xe2x80x9cBiaxially stretched,xe2x80x9d when used herein to describe a film, indicates that the film has been stretched in two different directions, a first direction and a second direction, in the plane of the film. Typically, but not always, the two directions are substantially perpendicular and are in the longitudinal or machine direction (xe2x80x9cMDxe2x80x9d) of the film (the direction in which the film is produced on a film-making machine) and the transverse direction (xe2x80x9cTDxe2x80x9d) of the film (the direction perpendicular to the MD of the film). The MD is sometimes referred to as the Longitudinal Direction (xe2x80x9cLDxe2x80x9d). Biaxially stretched films may be sequentially stretched, simultaneously stretched, or stretched by some combination of simultaneous and sequential stretching. xe2x80x9cSimultaneously biaxially stretched,xe2x80x9d when used herein to describe a film, indicates that significant portions of the stretching in each of the two directions are performed simultaneously. Unless context requires otherwise, the terms xe2x80x9corient,xe2x80x9d xe2x80x9cdraw,xe2x80x9d and xe2x80x9cstretchxe2x80x9d are used interchangeably throughout, as are the terms xe2x80x9coriented,xe2x80x9d xe2x80x9cdrawn,xe2x80x9d and xe2x80x9cstretched,xe2x80x9d and the terms xe2x80x9corienting,xe2x80x9d xe2x80x9cdrawing,xe2x80x9d and xe2x80x9cstretching.xe2x80x9d
The term xe2x80x9cstretch ratio,xe2x80x9d as used herein to describe a method of stretching or a stretched film, indicates the ratio of a linear dimension of a given portion of a stretched film to the linear dimension of the same portion prior to stretching. For example, in a stretched film having an MD stretch ratio (xe2x80x9cMDRxe2x80x9d) of 5:1, a given portion of unstretched film having a 1 cm linear measurement in the machine direction would have 5 cm measurement in the machine direction after stretch. In a stretched film having a TD stretch ratio (xe2x80x9cTDRxe2x80x9d) of 9:1, a given portion of unstretched film having a 1 cm linear measurement in the transverse direction would have 9 cm measurement in the transverse direction after stretch.
xe2x80x9cArea stretch ratio,xe2x80x9d as used herein, indicates the ratio of the area of a given portion of a stretched film to the area of the same portion prior to stretching. For example, in a biaxially stretched film having an overall area stretch ratio of 50:1, a given 1 cm2 portion of unstretched film would have an area of 50 cm2 after stretching.
The present invention provides a film suitable for use as an adhesive tape backing. The backing comprises isotactic polypropylene blended or mixed with at least a second polyolefin. The backing is biaxially stretched so as to provide a film having the following preferred characteristics:
A. Puncture energy up to 20 J/cm2 when 2.54 cm wide samples are tested by the method set out below, or up to 90 J/cm2 when 1.27 cm wide samples are tested by the method set out below.
B. Puncture elongation up to about 1.3 cm when 2.54 cm wide samples are tested by the method set out below, or up to 2.0 cm when 1.27 cm wide samples are tested by the method set out below.
C. Tensile strength up to 140 MPa in the MD.
In one preferred embodiment, the tape backing meets characteristics A, B, and C above. In another preferred embodiment, the tape backing meets condition A. In another preferred embodiment, the tape backing meets conditions A and B above. In yet another preferred embodiment, the tape backing meets conditions B and C above.
In one preferred embodiment, the puncture energy is up to 8 J/cm2, and in another embodiment up to 5 J/cm2, both with regard to testing 2.54 cm wide samples.
In another preferred embodiment, the tensile strength is up to 100 MPa, and in another preferred embodiment, up to 50 MPa.
In another preferred embodiment, the second component comprises polyethylene, polybutylene, or syndiotactic polypropylene.
In another preferred embodiment, the backing comprises 20% to 40% of said second component.
In another preferred embodiment, the backing comprises a monolayer.
In another preferred embodiment, the isotactic polypropylene has a first melting point, and the second component has a second melting point lower than the first melting point and above 100xc2x0 C. More preferably, the second melting point is between about 120xc2x0 C. and 155xc2x0 C.
In another preferred embodiment, the backing has an area stretch ratio of at least 36:1.
In another preferred embodiment, the backing has an MDR of at least 4:1.
In another preferred embodiment, when the backing is severed on a serrated plastic cutting blade, the backing exhibits a serrated edge that closely follows the contour of the cutting blade.
In another preferred embodiment, the backing has been simultaneously biaxially stretched.
Tape backings of the present invention may be hand tearable. It has been found that when the backing has a puncture energy of up to 8 J/cm2 (as tested on a 2.54 cm wide sample) and a tensile strength of up to 100 MPa, the tape backing is moderately easy to tear by hand. When the backing has a puncture energy of up to 5 J/cm2 (as tested on a 2.54 cm wide sample) and a tensile strength of up to 50 MPa, the tape backing is easy to tear by hand.
The present invention provides films described above, tape backings made from such films, tapes including the backings, and methods of making the films, backings, and tapes.